Transistor in simple words. Strengthening in other areas of technology and nature

13. Device and principle of operation of transistors

Depending on the principle of operation and design features, transistors are divided into two large classes: bipolar and field.

Bipolar transistors are semiconductor devices with two or more interacting electrical p-n junctions and three or more terminals, the amplifying properties of which are due to the phenomena of injection and extraction of minor charge carriers.

Currently, bipolar transistors with two p-n junctions are widely used, to which this term is most often referred. They consist of alternating regions (layers) of a semiconductor having electrical conductivities of different types. Depending on the type of electrical conductivity of the outer layers, transistors are distinguished r-p-r and n-p-n-types.

Transistors in which p-n junctions are created at the contact surfaces of the semiconductor layers are called planar

bipolar transistor is a semiconductor crystal consisting of three layers with alternating conductivity and equipped with three leads (electrodes) for connection to an external circuit.

On fig. 1.5, and b shows the circuit designation of two types of transistors p-p-p-type And p-r-p- type . The outer layers are called emitterum(E) and collector(K), between them is base(B). The three-layer structure has two p-n junctions: emitter junction between emitter and base and collector junction between base and collector. As the source material for transistors, germanium or silicon is used.

When manufacturing a transistor, two conditions must be met:

base thickness (distance between emitter and coll-

lecturer transitions) should be small compared to the mean free path of charge carriers;

2) the concentration of impurities (and main charge carriers) in the emitter must be significantly higher than in the base (N a >> N D in r-p-r transistor).

Consider the principle of operation r-p-r transistor.

The transistor is connected in series with the load resistance Rk in the circuit of the collector voltage source E to . The control EMF is applied to the input of the transistor E B", as shown in Fig. 1.6, a. Such a turn on of the transistor when the input ( E B , R B ) and day off ( E TO , R TO ) chains have a common point - the emitter, is the most common and is called the inclusion with common emitter(OE).

In the absence of stress (E B =0, E TO\u003d 0) the emitter and collector junctions are in a state of equilibrium, the currents through them are equal to zero. Both transitions have a double electric layer, consisting of impurity ions, and a potential barrier  o, which is different for each of the transitions. The distribution of potentials in the transistor in the absence of voltages is shown in fig. 1.6b dashed line.

Polarity of external sources E B and E TO is chosen such that there is a forward voltage at the emitter junction (minus the source E B applied to the base, plus - to the emitter), and at the collector junction - reverse voltage (minus source E TO- to the collector, plus - to the emitter), and the voltage |Uke|> |Ube| (voltage at the collector junction Ukb \u003d Uke-Ube) With this inclusion of sources E B and E TO the potential distribution in the transistor has the form shown in fig. .1.6, b solid line. The potential barrier of the forward-biased emitter junction decreases, and the potential barrier increases at the collector junction. As a result of applying a forward voltage to the emitter junction, enhanced diffusion (injection) of holes from the emitter into the base begins. The electronic component of the diffusion current through the emitter junction can be neglected, since R R >>p P , since the condition was stipulated above N BUT >>N D . Thus, the emitter current I E \u003d I Edif R. Under the influence of diffusion forces, as a result of a concentration drop along the base, holes move from the emitter to the collector. Since the base in the transistor is thin, most of the holes injected by the emitter reach the collector junction without entering the recombination centers. These holes are captured by the field of the collector junction, shifted in the opposite direction, since this field is accelerating for minority carriers - holes in the n-type base. The current of holes that got from the emitter to the collector is closed through an external circuit, the source E TO . With an increase in the emitter current by I E, the collector current will increase by I K = I E. Due to the low probability of recombination in a thin base, the emitter current transfer coefficient  \u003d I K / I E \u003d 0.9-0.99 .

A small part of the holes injected by the emitter enters the recombination centers and disappears, recombining with electrons. The charge of these holes remains in the base, and to restore the charge neutrality of the base from the external circuit due to the source Ev electrons enter the base. Therefore, the base current is a recombination current I rec \u003d I E (1-) In addition to the indicated main components of the transistor current, it is necessary to take into account the possibility of transition of minority carriers that arise in the base and collector as a result of carrier generation through a collector junction to which a reverse voltage is applied. This small current (the passage of holes from the base to the collector and electrons from the collector to the base) is similar to the reverse current r-p transition, also called collector junction reverse current or thermal current and is designated I kbo (Fig. 1.6, a)

FETs- semiconductor devices that practically do not consume current from the input circuit.

Field-effect transistors are divided into two types, differing from each other in the principle of operation: a) with r-p transition; b) MDP-type.

. 1.6.1. FETs withr-p transition have a structure, the section of which is shown in Fig. 1.9, a. A layer with p-type conductivity is called channel, it has two outputs to the external circuit: FROM- stock And AND- source. Layers with conductivity type P, surrounding the channel are interconnected and have an output to an external circuit, called shutter 3. Connection of voltage sources to the device is shown in fig. 1.9, a, in fig. 1.9.6 shows the circuit designation of a field effect transistor with r-p junction and p-type channel. There are also field-effect transistors with an n-type channel, their designation is shown in fig. 1.9 in, the principle of operation is similar, but the directions of the currents and the polarity of the applied voltages are opposite.

Consider the principle of operation of a field-effect transistor with a p-type channel. On fig. 1.9 G the family of drain (output) characteristics of this device is given Iс=f(Uс) at Uз=const.

With a control voltage Uzi = 0 and a voltage source is connected between the drain and the source Usi A current flows through the channel, which depends on the resistance of the channel. Voltage Us applied uniformly along the length of the channel, this voltage causes a reverse bias r-p transition between the p-type channel and the n-layer, with the highest reverse voltage across r-p transition exists in the region adjacent to the drain, and near the source r-p transition is in equilibrium. With increasing voltage Usi electric double layer region r-p transition, depleted of mobile charge carriers, will expand, as shown in Fig. 1.10, but. The expansion of the junction is especially pronounced near the drain, where the reverse voltage at the junction is greater. Extension r-p transition leads to a narrowing of the current-conducting channel of the transistor, and the resistance of the channel increases. Due to the increase in channel resistance with increasing Us, the drain characteristic of the field-effect transistor has a non-linear character (Fig. 1.9, d). At some voltage Usi borders r-p transitions are closed (dotted line in Fig. 1.10, a), and the increase in current Ic with increasing Ucb stops.

When a positive voltage is applied to the gate Uzi>0 r-p the transition is even more shifted to the reverse voltage region, the width of the transition increases, as shown in Fig. 1.10.6. As a result, the current-conducting channel narrows and the current Ic decreases. Thus, increasing the voltage Uzi. it is possible to reduce Ic, which can be seen from the consideration of Fig. 1.9 G. At a certain Uzi called cutoff voltage, there is practically no drain current. The ratio of the change in the drain current I C to the change in voltage between the gate and the source Uzi at Usi = const that caused it is called slope:S = I C /Uzi at Usi = const

Unlike bipolar transistors, FETs are voltage controlled and only a small thermal current Iz flows through the gate circuit. r-p junction under reverse voltage.

The principle of semiconductor control of electric current was known as early as the beginning of the 20th century. Despite the fact that engineers working in the fields of radio electronics knew how the transistor worked, they continued to design devices based on vacuum tubes. The reason for such distrust of semiconductor triodes was the imperfection of the first point transistors. The family of germanium transistors did not differ in the stability of their characteristics and was highly dependent on temperature conditions.

Serious competition for vacuum tubes was made by monolithic silicon transistors only at the end of the 50s. Since that time, the electronic industry began to develop rapidly, and compact semiconductor triodes actively replaced energy-intensive lamps from the circuits of electronic devices. With the advent of integrated circuits, where the number of transistors can reach billions, semiconductor electronics has won a convincing victory in the struggle for the miniaturization of devices.

What is a transistor?

In the modern sense, a transistor is called a semiconductor radio element designed to change the parameters of an electric current and control it. A conventional semiconductor triode has three outputs: a base to which control signals are applied, an emitter and a collector. There are also high power composite transistors.

The size scale of semiconductor devices is striking - from a few nanometers (unpackaged elements used in microcircuits) to centimeters in diameter of powerful transistors designed for power plants and industrial equipment. Reverse voltages of industrial triodes can reach up to 1000 V.

Device

Structurally, the triode consists of semiconductor layers enclosed in a housing. Semiconductors are materials based on silicon, germanium, gallium arsenide and other chemical elements. Today, research is being carried out that prepares some types of polymers, and even carbon nanotubes, for the role of semiconductor materials. Apparently in the near future we will learn about the new properties of graphene field-effect transistors.

Previously, semiconductor crystals were located in metal cases in the form of hats with three legs. This design was typical for point transistors.

Today, the designs of most flat, including silicon, semiconductor devices are made on the basis of a single crystal doped in certain parts. They are pressed into plastic, glass-metal or ceramic-metal housings. Some of them have protruding metal plates for heat dissipation, which are mounted on radiators.

The electrodes of modern transistors are arranged in one row. This arrangement of legs is convenient for automatic board assembly. The terminals are not marked on the housings. The type of electrode is determined by reference books or by measurements.

For transistors, semiconductor crystals with different structures are used, such as p-n-p or n-p-n. They differ in the polarity of the voltage on the electrodes.

Schematically, the structure of a transistor can be represented as two semiconductor diodes separated by an additional layer. (See figure 1). It is the presence of this layer that makes it possible to control the conductivity of the semiconductor triode.

Rice. 1. The structure of transistors

Figure 1 schematically shows the structure of bipolar triodes. There is another class of field-effect transistors, which will be discussed below.

Basic principle of operation

At rest, no current flows between the collector and emitter of a bipolar triode. The resistance of the emitter junction, which arises as a result of the interaction of the layers, prevents the electric current. To turn on the transistor, it is required to apply a slight voltage to its base.

Figure 2 shows a diagram explaining how a triode works.

Rice. 2. Working principle

By controlling the base currents, you can turn the device on and off. If an analog signal is applied to the base, it will change the amplitude of the output currents. In this case, the output signal will exactly repeat the oscillation frequency at the base electrode. In other words, there will be an amplification of the electrical signal received at the input.

Thus, semiconductor triodes can operate in the mode of electronic keys or in the mode of amplifying input signals.

The operation of the device in the electronic key mode can be understood from Figure 3.

Rice. 3. Triode in key mode

Designation on the diagrams

Common notation: "VT" or "Q" followed by a positional index. For example, VT 3. In earlier diagrams, obsolete designations can be found: “T”, “PP” or “PT”. The transistor is depicted as symbolic lines indicating the corresponding electrodes, circled or not. The direction of the current in the emitter is indicated by an arrow.

Figure 4 shows a ULF circuit, in which transistors are labeled in a new way, and Figure 5 shows schematic representations of different types of field-effect transistors.

Rice. 4. An example of a ULF circuit on triodes

Types of transistors

According to the principle of operation and structure, semiconductor triodes are distinguished:

• field;
• bipolar;
• combined.

These transistors perform the same functions, but there are differences in the principle of their operation.

field

This type of triode is also called unipolar, because of the electrical properties - they have a current of only one polarity. According to the structure and type of control, these devices are divided into 3 types:

1. Transistors with a control p-n junction (Fig. 6).
2. With an insulated gate (there are with a built-in or with an induced channel).
3. MDP, with the structure: metal-dielectric-conductor.

A distinctive feature of an insulated gate is the presence of a dielectric between it and the channel.

Parts are very sensitive to static electricity.

Field triode circuits are shown in Figure 5.

Rice. 5. Field-effect transistors
Rice. 6. Photo of a real field triode

Pay attention to the name of the electrodes: drain, source and gate.

FETs consume very little power. They can last over a year on a small battery or accumulator. Therefore, they have found wide application in modern electronic devices such as remote controls, mobile gadgets, etc.

Bipolar

Much has been said about this type of transistor in the subsection “Basic principle of operation”. We only note that the device received the name "Bipolar" because of the ability to pass charges of opposite signs through one channel. Their feature is a low output impedance.

Transistors amplify signals and act as switching devices. A sufficiently powerful load can be included in the collector circuit. Due to the large collector current, the load resistance can be reduced.

We will consider in more detail about the structure and principle of operation below.

Combined

In order to achieve certain electrical parameters from the use of one discrete element, transistor developers invent combined designs. Among them are:

• with resistors embedded and their circuit;
• combinations of two triodes (identical or different structures) in one case;
• lambda diodes - a combination of two field triodes forming a section with negative resistance;
• constructions in which an insulated gate field triode controls a bipolar triode (used to control electric motors).

Combined transistors are, in fact, an elementary microcircuit in one package.

How does a bipolar transistor work? Instructions for dummies

The operation of bipolar transistors is based on the properties of semiconductors and their combinations. To understand the principle of operation of triodes, we will deal with the behavior of semiconductors in electrical circuits.

Semiconductors.

Some crystals, such as silicon, germanium, etc., are dielectrics. But they have one feature - if you add certain impurities, they become conductors with special properties.

Some additives (donors) lead to the appearance of free electrons, while others (acceptors) form “holes”.

If, for example, silicon is doped with phosphorus (donor), then we get a semiconductor with an excess of electrons (n-Si structure). When boron is added (acceptor), doped silicon will become a hole-conducting semiconductor (p-Si), that is, positively charged ions will predominate in its structure.

Unidirectional conduction.

Let's conduct a thought experiment: let's connect two heterogeneous semiconductors to a power source and bring current to our design. Something unexpected will happen. If you connect the negative wire to an n-type crystal, the circuit will close. However, when we reverse the polarity, there will be no electricity in the circuit. Why it happens?

As a result of the connection of crystals with different types of conductivity, a region with a p-n junction is formed between them. Part of the electrons (charge carriers) from the n-type crystal will flow into a crystal with hole conductivity and recombine holes in the contact zone.

As a result, uncompensated charges arise: in the n-type region - from negative ions, and in the p-type region from positive ones. The potential difference reaches a value of 0.3 to 0.6 V.

The relationship between voltage and impurity concentration can be expressed by the formula:

φ= V T*ln( N n* Np)/n 2 i , where

V T – thermodynamic stress value, N n And Np – the concentration of electrons and holes, respectively, and n i denotes the intrinsic concentration.

When connecting a plus to a p-conductor, and a minus to an n-type semiconductor, electric charges will overcome the barrier, since their movement will be directed against the electric field inside the p-n junction. In this case, the transition is open. But if the poles are reversed, the transition will be closed. Hence the conclusion: the p-n junction forms one-way conduction. This property is used in the design of diodes.

From diode to transistor.

Let's complicate the experiment. Let's add one more layer between two semiconductors with the same structures. For example, between p-type silicon wafers, we insert a conductive layer (n-Si). It is not difficult to guess what will happen in the contact zones. By analogy with the process described above, regions with p-n junctions are formed that block the movement of electric charges between the emitter and collector, regardless of the polarity of the current.

The most interesting thing happens when we apply a slight voltage to the interlayer (base). In our case, we apply a current with a negative sign. As in the case of a diode, an emitter-base circuit is formed, through which current will flow. At the same time, the layer will begin to be saturated with holes, which will lead to hole conduction between the emitter and collector.

Look at Figure 7. It shows that positive ions have filled the entire space of our conditional design and now nothing interferes with the conduction of current. We have obtained a visual model of a p-n-p bipolar transistor.

Rice. 7. The principle of operation of the triode

When the base is de-energized, the transistor very quickly returns to its original state and the collector junction closes.

The device can also operate in amplifying mode.

The collector current is directly proportional to the base current. : Ito= ß* IB , where ß – current gain, IB – base current.

If you change the value of the control current, then the intensity of the formation of holes on the base will change, which will entail a proportional change in the amplitude of the output voltage, while maintaining the frequency of the signal. This principle is used to amplify signals.

By applying weak pulses to the base, at the output we get the same amplification frequency, but with a much larger amplitude (set by the voltage applied to the collector-emitter circuit).

NPN transistors work in a similar way. Only the polarity of the voltages changes. Devices with an n-p-n structure have direct conduction. P-n-p type transistors have reverse conductivity.

It remains to add that a semiconductor crystal reacts in a similar way to the ultraviolet spectrum of light. By turning the photon flux on and off, or by adjusting its intensity, one can control the operation of the triode or change the resistance of a semiconductor resistor.

Bipolar transistor switching circuits

Circuit engineers use the following connection schemes: with a common base, common emitter electrodes and switching on with a common collector (Fig. 8).

Rice. 8. Wiring diagrams for bipolar transistors

For amplifiers with a common base is typical:

• low input impedance, which does not exceed 100 ohms;
• good temperature properties and frequency characteristics of the triode;
• high allowable voltage;
• requires two different power supplies.

Common emitter circuits have:

• high current and voltage gains;
• low power gain;
• inversion of the output voltage relative to the input.

With this connection, one power supply is sufficient.

The connection scheme according to the "common collector" principle provides:

• high input and low output impedance;
• low voltage gain (< 1).

How does a field effect transistor work? Explanation for dummies

The structure of a field-effect transistor differs from a bipolar one in that the current in it does not cross the p-n junction zones. The charges move along an adjustable area called the gate. Gate bandwidth is regulated by voltage.

The space of the p-n zone decreases or increases under the action of an electric field (see Fig. 9). Accordingly, the number of free charge carriers changes - from complete destruction to ultimate saturation. As a result of such an impact on the gate, the current at the drain electrodes (contacts that output the processed current) is regulated. The incoming current flows through the source contacts.

Figure 9. FET with p-n junction

Field triodes with a built-in and induced channel work on a similar principle. You saw their schemes in Figure 5.

FET switching circuits

In practice, connection schemes are used by analogy with a bipolar triode:

• with a common source - gives a large amplification of current and power;
• common-gate circuits providing low input impedance and low gain (of limited use);
• common-drain circuits that work in the same way as common-emitter circuits.

Figure 10 shows various wiring diagrams.

Rice. 10. Image of field triode connection diagrams

Almost every circuit is capable of operating at very low input voltages.

Video explaining the principle of operation of the transistor in simple terms

How a diode works

This is such a tricky thing that passes current only in one direction. It can be compared to a nipple. It is used, for example, in rectifiers, when alternating current is made direct. Or when it is necessary to separate the reverse voltage from the direct one. Look at the programmer circuit (where there was an example with a divider). You see there are diodes, what do you think, why? And everything is simple. For the microcontroller, the logic levels are 0 and 5 volts, and for the COM port, one is minus 12 volts, and zero is plus 12 volts. Here the diode cuts off this minus 12, forming 0 volts. And since the conductivity of the diode in the forward direction is not ideal (it generally depends on the applied forward voltage, the larger it is, the better the diode conducts current), then about 0.5-0.7 volts will drop on its resistance, the remainder, being divided by resistors in two, will be approximately 5.5 volts, which is within the limits of the controller.
The terminals of a diode are called an anode and a cathode. Current flows from the anode to the cathode. It is very simple to remember where which conclusion is: on the symbol, the arrow and the stick from the side to as if drawing a letter TO here look - TO|—. K = Cathode! And on the part, the cathode is indicated by a strip or a dot.

There is another interesting type of diode - zener diode. I used it in one of the previous articles. Its peculiarity is that in the forward direction it works like a conventional diode, but in the reverse direction it breaks off at some voltage, for example, at 3.3 volts. Similar to a steam boiler pressure relief valve that opens when pressure is exceeded and bleeds off excess steam. Zener diodes are used when they want to get a voltage of a given value, regardless of the input voltages. This can be, for example, a reference value against which the input signal is compared. They can cut the incoming signal to the desired value or use it as protection. In my circuits, I often put a 5.5 volt zener diode to power the controller, so that if something happens, if the voltage jumps sharply, this zener diode bleeds the excess through itself. There is also such a beast as a suppressor. The same zener diode, only much more powerful and often bidirectional. Used for nutrition protection.

Transistor.

A terrible thing, as a child I could not understand how it works, but everything turned out to be simple.
In general, a transistor can be compared to a controlled valve, where we control the most powerful flow with a tiny effort. He turned the handle a little and tons of shit rushed through the pipes, opened it harder and now everything around was choked in sewage. Those. The output is proportional to the input multiplied by some value. This value is the gain.
These devices are divided into field and bipolar.
The bipolar transistor has emitter, collector And base(see drawing of symbol). The emitter is with an arrow, the base is designated as a straight platform between the emitter and the collector. There is a large payload current between emitter and collector, the direction of the current is determined by the arrow on the emitter. But between the base and the emitter there is a small control current. Roughly speaking, the magnitude of the control current affects the resistance between the collector and emitter. Bipolar transistors are of two types: p-n-p And n-p-n the fundamental difference is only in the direction of the current through them.

A field-effect transistor differs from a bipolar one in that in it the channel resistance between the source and drain is no longer determined by the current, but by the gate voltage. Recently, field-effect transistors have gained immense popularity (all microprocessors are built on them), because. microscopic currents flow in them, voltage plays a decisive role, which means that losses and heat generation are minimal.

In short, the transistor will allow you a weak signal, for example from the foot of the microcontroller,. If the amplification of one transistor is not enough, then they can be connected in cascades - one after the other, more and more powerful. And sometimes one mighty field MOSFET transistor. Look, for example, how a vibrating alert is controlled in cell phone circuits. There, the output from the processor goes to the gate of the power MOSFET key.

Every year there are more and more electronic devices, and they often break down. A lot of money is spent on repairs, sometimes reaching up to 50 percent of the cost of the device. And, annoyingly, some of these breakdowns could be fixed by yourself, having a basic knowledge of how the transistor works. Why he? It is transistors that most often fail.

Types of transistor

To make it easier to understand the operation of the transistor, you need to have an idea about it. It is a semiconductor, which indicates its ability to conduct current in one direction and not pass in the other. To achieve these characteristics, different manufacturing methods are used. All these devices are divided into two groups according to the nature of their work.:

1. bipolar
2. polar

Although both belong to the same class - transistors, the processes occurring in them are very different.

Bipolar

The movement of electrons in a closed circuit is called electric current. Roughly speaking, the more electrons, the more current. If an atom donates electrons, it becomes positively charged, and vice versa, by attracting extra electrons, it becomes negatively charged.

When impurities are added to silicon and germanium, they become a necessary material from which bipolar transistors are made.

Bipolar devices are electronic devices that consist of two layers with different charges.. Moreover, the two extreme ones have the same charge. The layer that has a positive charge is called “p”, and the negative one is called “n”. In this regard, the following types are distinguished:

• p-n-p
• n-p-n

The boundary between these layers is called a transition.. The inner region, divided by two transitions, is called the base. The two outer regions are called emitter and collector. The monocrystal is made in such a way that one outer region transfers energy carriers to the base and is called the emitter. The other outer region picks up these media and is called the collector.

On the electrical circuit, a bipolar transistor is indicated in the form of a circle, inside which a dash is drawn, and three straight lines approach it. One fits at an angle of 90 degrees and marks the base, the other two at an angle. The one that has an arrow indicates the emitter, the other - the collector. The device itself, as a rule, has three conclusions corresponding to these areas.

Field

Another type is called field or unipolar. Unlike the bipolar p-n junction, it works differently. Its single crystal has a homogeneous composition. The channel through which energy carriers move can be hole or electronic. In a hole carrier, positively charged immobile ions are present, in an electronic one, negatively charged ones. These channels are also referred to as "p" and "n" respectively.

Around and almost along the entire length of this channel, ions of opposite polarity are injected and implanted.. This area is called the gate, and it regulates the conductivity of the channel. The edge of the channel through which charged particles enter the crystal is called the source, and through which they exit - the drain.

To improve the electrical characteristics, a dielectric was added between the metal channel and the gate. If we classify transistors by structure, we can distinguish two families:

• MIS (these include MOS - metal-oxide-conductor)

MDP stands for Metal-Dielectric-Conductor. This is field. The new JGBT transistor combines the advantages of bipolar, but has an insulated gate.

Operating principle

One of the complex radio elements is a transistor. Its working principle is as follows.:

• adjustment
• gain
• generation

Bipolar have more power and can work with high frequencies. However, if you need a wide range of amplification, then you cannot do without a field one.

Field work

Consider how a transistor works. For beginner radio amateurs, it is difficult to understand all these transitions. To show the principle of operation of a transistor in simple terms, let's pay attention to the following example..

A valve-type faucet is able to change the pressure of water very smoothly. This is achieved by gradually changing the orifice. The operation of a field-effect transistor is based on the same principle.

The gate surrounds the passageway. When a blocking voltage is applied to it, the electric field, as it were, squeezes the passage, thereby reducing the flow of charged particles. Just as when closing a faucet, a little effort is required, so the shutter power, compared to the main channel, is very small. The similarity is also in the fact that with small changes in the gate voltage, the passage cross section also changes slightly.

How Bipolar Works

The operation of a bipolar device is somewhat different from that of a field device.. First of all, the method of controlling the movement of charged particles differs. In the field, an electric field is used, in the bipolar - the current between the base and the emitter.

Depending on the type of device, the emitter arrow in the diagram will either be directed towards the base, then it is the p-n-p type, or away from the base, then it is n-p-n. When a voltage of the same name is connected to these terminals (“p” is connected to “+”, and “n” is connected to “-”), a current arises in the emitter-base circuit. More charge carriers appear in the base, and the greater the current in this circuit, the more they become.

A reverse voltage is applied to the collector, i.e., “-” is connected to “p”, and “+” is connected to “n”. Since there is a potential difference between the emitter and collector, a current appears between these terminals. It will be the greater, the more charge carriers there are in the base.

When a power supply of the opposite sign is connected to the emitter and base, the current stops, the transistor closes. What will help to better understand the operation of the transistor? For dummies, it is important to understand one truth. If the emitter-base junction is open (forward voltage is applied), then the device itself is open, otherwise it is closed.

Precautionary measures

Field-effect transistors are very sensitive to high voltage. When working with them, it is necessary to prevent the possibility of static voltage falling on them. This can be achieved by wearing a grounded bracelet. When selecting an analog, it is important to consider not only the operating voltage, but also the permissible current. And if the device operates in frequency mode, then its frequency.

Transistors are active components and are used throughout electronic circuits as amplifiers and switching devices (transistor switches). As amplifying devices, they are used in high and low frequency devices, signal generators, modulators, detectors, and many other circuits. In digital circuits, in switching power supplies and controlled electric drives, they serve as keys.

Bipolar transistors

This is the name of the most common type of transistor. They are divided into npn and pnp types. The material for them is most often silicon or germanium. At first, transistors were made from germanium, but they were very sensitive to temperature. Silicon devices are much more resistant to its fluctuations and cheaper to manufacture.

Various bipolar transistors are shown in the photo below.

Low-power devices are located in small plastic rectangular or metal cylindrical cases. They have three outputs: for the base (B), emitter (E) and collector (K). Each of them is connected to one of the three layers of silicon with either n-conductivity (the current is formed by free electrons) or p-type (the current is formed by the so-called positively charged “holes”) that make up the structure of the transistor.

How is a bipolar transistor arranged?

The principles of operation of the transistor must be studied, starting with its device. Consider the structure of an npn transistor, which is shown in the figure below.

As you can see, it contains three layers: two with n-type conductivity and one with p-type. The type of conductivity of the layers is determined by the degree of doping with special impurities of various parts of the silicon crystal. The n-type emitter is very heavily doped in order to get a lot of free electrons as the main current carriers. The very thin p-type base is lightly doped with impurities and has high resistance, while the n-type collector is very heavily doped to give it low resistance.

How a transistor works

The best way to get to know them is by experimentation. Below is a diagram of a simple circuit.

It uses a power transistor to control the light bulb. You will also need a battery, a small flashlight bulb of about 4.5 V / 0.3 A, a variable resistor potentiometer (5K) and a 470 ohm resistor. These components must be connected as shown in the figure to the right of the diagram.

Turn the potentiometer slider to the lowest position. This will lower the base voltage (between base and ground) to zero volts (U BE = 0). The lamp does not glow, which means there is no current through the transistor.

If you now turn the handle from its lower position, then U BE gradually increases. When it reaches 0.6 V, current begins to flow into the base of the transistor, and the lamp begins to glow. When the handle is moved further, the voltage U BE remains at 0.6 V, but the base current increases and this increases the current through the collector-emitter circuit. If the handle is moved to the up position, the voltage at the base will increase slightly to 0.75 V, but the current will increase significantly and the lamp will glow brightly.

And if you measure the currents of the transistor?

If we include an ammeter between the collector (C) and the lamp (to measure IC), another ammeter between the base (B) and the potentiometer (to measure IB), and a voltmeter between common and base, and repeat the whole experiment, we can get some interesting data. When the potentiometer knob is in its lowest position, U BE is 0 V, as are the currents I C and I B . When the handle is moved, these values ​​increase until the light starts to glow, when they are equal: U BE = 0.6 V, I B = 0.8 mA and I C = 36 mA.

As a result, we get the following principles of transistor operation from this experiment: in the absence of a positive (for npn-type) bias voltage on the base, the currents through its terminals are zero, and in the presence of base voltage and current, their changes affect the current in the collector-emitter circuit.

What happens when the transistor is turned on

During normal operation, the voltage applied to the base-emitter junction is distributed so that the potential of the base (p-type) is approximately 0.6 V higher than that of the emitter (n-type). At the same time, a forward voltage is applied to this junction, it is forward-biased and open for current flow from the base to the emitter.

A much higher voltage is applied across the base-collector junction, with the potential of the collector (n-type) being higher than that of the base (p-type). So a reverse voltage is applied to the junction and it is reverse biased. This results in a fairly thick electron depleted layer in the collector near the base when a supply voltage is applied across the transistor. As a result, no current flows through the collector-emitter circuit. The distribution of charges in the transition zones of the npn transistor is shown in the figure below.

What is the role of the base current?

How to make our electronic device work? The principle of operation of the transistor is to influence the base current on the state of the closed base-collector junction. When the base-emitter junction is forward biased, a small current will flow into the base. Here, its carriers are positively charged holes. They combine with electrons coming from the emitter to provide the current I BE . However, due to the fact that the emitter is very heavily doped, many more electrons flow from it to the base than are able to combine with holes. This means that there is a high concentration of electrons in the base, and most of them cross it and enter the electron-depleted collector layer. Here, they fall under the influence of a strong electric field applied to the base-collector junction, pass through the electron-depleted layer and the main volume of the collector to its output.

Changes in the current flowing into the base affect the number of electrons attracted from the emitter. Thus, the principles of transistor operation can be supplemented by the following statement: very small changes in the base current cause very large changes in the current flowing from the emitter to the collector, i.e. current amplification occurs.

Types of FETs

In English, they are designated FETs - Field Effect Transistors, which can be translated as "field effect transistors." Although there is a lot of confusion about the names for them, there are basically two main types of them:

1. With a control pn-junction. In English literature, they are referred to as JFET or Junction FET, which can be translated as "junction field effect transistor". Otherwise they are called JUGFET or Junction Unipolar Gate FET.

2. With an insulated gate (otherwise MOS or MIS transistors). In English, they are designated IGFET or Insulated Gate FET.

Outwardly, they are very similar to bipolar ones, which is confirmed by the photo below.

FET device

All field-effect transistors can be called UNIPOLE devices, because the charge carriers that form the current through them are of the only type for a given transistor - either electrons or "holes", but not both at the same time. This distinguishes the principle of operation of a field-effect transistor from a bipolar one, in which the current is generated simultaneously by both of these types of carriers.

Current carriers flow in FETs with a control pn junction along a layer of silicon without pn junctions, called a channel, with either n- or p-type conduction between two terminals, called "source" and "drain" - analogues of the emitter and collector or more precisely, the cathode and anode of the vacuum triode. The third output - a gate (analogous to a triode grid) - is connected to a silicon layer with a different type of conductivity than that of the source-drain channel. The structure of such a device is shown in the figure below.

How does a field effect transistor work? Its principle of operation is to control the cross section of the channel by applying voltage to the gate-channel junction. It is always reverse-biased, so the transistor draws almost no current through the gate circuit, while a bipolar device needs a certain amount of base current to operate. When the input voltage changes, the gate area can expand, blocking the source-drain channel until it is completely closed, thus controlling the drain current.